#[cfg(test)]
mod tests {
    #[test]
    fn string_def() {
        let _s1 = "i am a rust";
        let s1: &'static str = "i am a rust,too";
        let s2: String = s1.to_string();
        let s2: &String = &s2;

        let s3 = &s2[..];
        let s4 = &s2[..6];
        println!("{},{},{},{}",s1,s2,s3,s4);
    }
}